TJie Electric Discharge in a Magnetic Field. 245 



must so reject it. . . . Unless the hypothesis were axiomatic, 

 there could be no reason to adopt it to-day. Modern Physics 

 is perfectly capable of deducing a theory of elasticity from 

 the known laws of energy, without the aid of a subsidiary 

 hypothesis about intermolecular force, and being in that 

 position, it is bound to discard the hypothesis. Such a device 

 is merely a phase in the development of scientific thought, 

 and, having served its turn as a means of introducing gener- 

 ality into the subject, it must give place again to a still more 

 general method." It is of course quite obvious that the 

 experimental verification of Cauchy's relations would not 

 prove the molecular hypothesis ; but it would show the 

 contact-action conception to be inadequate. It would still 

 be open to any one with a preconceived idea as to what is 

 axiomatic to discard the former, but in doing so he would 

 find himself unable to account for known facts which had 

 been predicted by the aid of the discarded hypothesis. That 

 modern physics is capable of deducing a theory of elasticity 

 from the known laws of energy without the aid of a molecular 

 hypothesis is surely a mere assertion, if as doubtless is the 

 case, a satisfactory theory is meant. The experimental deter- 

 mination of any general relations among the 21 constants 

 would prove the theory in its present state to be inadequate 

 and unsatisfactory, and the verification of Cauchy's relations 

 would show that, while the molecular hypothesis must of 

 course be regarded as merely a phase in the development of 

 scientific thought, it has not yet quite completely served its 

 turn as an instrument of generalisation . 



XXY. The Electric Discliar ge in a Magnetic Field. 

 By Sir David Salomons*. 



THE study of the electric discharge in vacuo does not 

 appear to have been followed with the same energy as 

 has been applied to other branches of electrical science, 

 until quite recent times. I would venture to divide the 

 historic period of the subject into four, as follows : — 



(1) The time when Messrs. Warren De La Rue, Gassiot, 

 Spottiswoode, and Moulton were working upon the subject. 



(2) When Professor Crookes made his discoveries. 



(3) The still more recent investigations of Professor J. J. 

 Thomson. 



(4) The application to photography by employing the so- 

 called A'-rays which are abundantly produced by special forms 



of tubes. 



* Communicated by the Author. 



